A half-century after satellites began providing unprecedented views of the world’s oceans, a new generation of robots darting beneath the waves has begun filling in the vast gaps in oceanographic knowledge that extend all the way down to the seafloor and beyond.

Among the newest class of elite robotic explorers is an autonomous underwater vehicle, or AUV, which can swim like a fish or hover like a helicopter while precisely mapping seascapes. Another can glide for thousands of miles using only the ocean’s stored heat for power. And one is set to withstand crushing pressure while diving down nearly seven miles to the deepest ocean trench.

“The analogy I like to use is that there was a revolution when we got satellites up looking at the sea surface,” said Jack Barth, a professor of oceanography at Oregon State University. “We saw the tides and chlorophyll and algal blooms, but it allowed us to see only a few meters down from the surface. I think the autonomous vehicles used in fleets can paint us an underwater picture like we haven’t seen before.”

One of the newest stars, a free-swimming robot named Sentry, can travel at speeds of about 2 knots, or 2.3 miles per hour, almost twice as fast as its ‘90s predecessor, abbreviated ABE. If ABE resembles a miniature version of the Starship Enterprise from “Star Trek,” Sentry has been likened to an ocean sunfish, with its bright yellow, streamlined shape complemented by a pair of front and rear orange propeller-equipped fins.

“Not only can it slice through the water at a pretty good clip, but it can act like an underwater helicopter,” said Dan Fornari, director of the Deep Ocean Exploration Institute at Woods Hole Oceanographic Institution in Woods Hole, Mass.

Mapping the seafloorThe combination of swimming and hovering prowess made Sentry a natural choice for precisely mapping the seafloor at two spots off the coast of Oregon, a project completed last month as part of an ambitious plan to develop the nation’s first underwater observatories by 2014.

The Oceans Observatories Initiative, funded by the National Science Foundation and overseen by the Consortium for Ocean Leadership, aims to establish a sensor network extending dozens of miles into the ocean to the edge of the Juan de Fuca tectonic plate framing the Pacific Northwest coastline. Underwater fiber-optic cables would provide the electrical power and bandwidth to a suite of remotely operated sensors and high-definition video cameras designed to monitor marine life, chemical reactions and natural phenomena such as tsunamis, earthquakes, hydrothermal vents and underwater volcanoes.

One site, named Axial Seamount, offers the largest active underwater volcano east of Hawaii and north of Mexico. Another, known as Hydrate Ridge, contains sizeable deposits of methane hydrates, where energy-rich methane has been trapped within marine sediments in an ice-like hydrate form.

On its research debut, the one-of-a-kind Sentry produced maps of both sites accurate to within 3.5 feet while maneuvering about 250 feet above cliffs, basins and other features on the seafloor. Following a pattern reminiscent of a lawnmower working its way across a yard, Sentry surveyed more than 20 square miles in all over the course of six dives.

John Delaney, a professor of oceanography at the University of Washington and the chief scientist for last month’s mapping expedition, said the precise plotting was critical for ensuring that the observatories are positioned where the action is but not so close that the equipment is liable to be destroyed by landslides, eruptions or quakes. Like an octopus, arms of fiber-optic cables could extend out from safely positioned central nodes toward more interesting, if unstable, terrain. Even if severed by a landslide, an arm that ventured too close to a hotspot would leave the central network intact and functional.

Similarly, fleets of AUVs could serve as mobile assistants dispatched by a central underwater lab to investigate a new seafloor disruption or phenomenon and then dock with a fixed installation to upload data from their missions.

In a sneak preview of what the future might look like, Delaney showed an animation of how such a fleet could be released from an underwater garage and collect samples, such as a bloom of microbes jetting through the seafloor in the aftermath of an earthquake. After the vehicles relay those samples to a surface station, a pilot-less drone could be sent to deliver the packets to land-based researchers for further study.

As for the kinds of sensors that might be wielded by the observatories and their roving robotic helpers, Delaney and his colleagues have a wish-list nearly three pages long, from seismometers and pressure gauges to still-in-development machines that can analyze chemical samples or even sequence genes.

For geologists like him, he said, being able to tap the new AUVs as faster and more efficient mini-labs means being able to ask the kind of questions that can only come from an especially close vantage: How do underwater volcanoes support life? How do they concentrate minerals like copper, zinc, lead, gold and silver? And what do they say about the Earth’s formation?

After all, Delaney said, “If you want to understand the human body, you don’t do it by watching people walk across the street.” Eventually, the ocean’s robot-aided close-up may be accessible to anyone regardless of location, a point he likes to emphasize with his mantra, “Interacting with the Ocean From Anywhere on Earth.”

Making the ocean more accessible
Such plans are not without their pitfalls. Reached by e-mail during a joint ocean expedition with Chinese scientists, lead Sentry engineer Dana Yoerger said an autonomous vehicle “only has the limited brainpower we’ve programmed into it. Sometimes with an AUV, an A-minus can be a failing grade.”

Sentry has comfortably passed its initial tests, said Yoerger, but another AUV powered by the ocean has demonstrated both the potential and peril for the new explorers.

A new generation of thermal-powered glider can travel more than nine miles a day and descend about three-fourths of a mile below the surface, using only the ocean’s stored heat, said Benjamin Hodges, a research associate at Woods Hole Oceanographic Institution.

To propel itself through the water, the glider relies on wax that melts and expands at a temperature of about 10 degrees Celsius, or 50 degrees Fahrenheit. As the glider ascends from colder, deeper water to the warmer surface water, the melting wax compresses nitrogen gas in a bottle within the glider to a pressure of 3,000 pounds per square inch, or the amount of pressure typically encountered in hydraulic systems on commercial airplanes. On its next dive, when the glider reaches its maximum depth and needs to ascend, it uses the compressed nitrogen to force oil into an external bladder, increasing the glider’s buoyancy and causing it to rise. Then the cycle repeats.

After completing a four-month mission with no problems, Hodges said in an e-mail, the thermal glider vanished at sea about two months into its second trip.

“We don’t know what happened,” he said, suggesting a ship-strike or entanglement in fishing gear as potential culprits. Another glider is now being built by East Falmouth, Mass.-based Teledyne Webb Research and will likely be tested in the Caribbean.

Despite the snag, researchers say the upper ocean has become far more accessible through sophisticated gliders and other autonomous vehicles that passively drift with the currents. At Oregon State University, for example, Barth and his colleagues have routinely deployed battery-powered gliders to measure parameters such as the dissolved oxygen content, a critical factor for marine life. As part of the Ocean Observatory Initiative, he said, the collaborators hope to deploy a fleet of six gliders to patrol the waters around the stationary observatories and answer questions like, “What does the neighborhood look like in the region of these very intensive measurements?”

Diving into the depthsNereus, a hybrid submersible still in the testing phase, could build upon that promise with its ability to dive down to nearly seven miles, or the depth of the Mariana Trench east of Japan.

“This is an oceanographer’s dream transformer,” Fornari said, referring to the vehicle’s unique ability to change from an autonomous to remotely operated vehicle. Nereus boasts an ultra-thin ceramic coating that resists the crushing pressure of the deep ocean but doesn’t saddle the vehicle with too much weight. LED lights will provide illumination and 20 kilometers of monofilament fishing line-like microfiber will transmit data to the surface.

So far, the vehicle has been pressure-tested to an equivalent of what it would encounter in the Mariana Trench and field-tested in depths of 2.5 to 3 miles off the coast of Hawaii. Nereus is being readied to explore the trenches of the western Pacific Ocean in early 2009, and if all goes well, it will move on to the Cayman Trough to search for new vent-fields along the world’s deepest mid-ocean ridge.

Chris German, the chief scientist for the National Deep Submergence Facility at Woods Hole and a fellow member of the joint U.S.-China expedition, noted that the vast majority of the ocean floor has yet to be investigated. Human-occupied vehicles are scarce, he said in an e-mail, and remotely operated vehicles still require a significant investment to deploy to far-flung parts of the world.

“So you really need to know as much as possible about where you are going before you make that kind of commitment,” he said. “That is where we have found AUVs such a great asset.”

Nereus, he said, will not only locate, map and photograph the seafloor but also collect chemical, biological and geological samples.

With the latest class of underwater robots to guide them, German and his colleagues are hoping that such feats — once only dreamed about — will soon become little more than routine aspects of the new era in ocean exploration.